When programming and setting up a machine tool, operating errors may result in the tool or the work spindle colliding with a work piece arranged in the processing chamber during its displacement in the x, y or z direction. This causes damages to bearings, deformations of shafts, crack formations or material fractures on tool, spindle or mounting components. These damages are associated with significant repair costs, particularly when optically invisible damages are overlooked and the further processing of the work piece is continued with a damaged system.
Mechanical safety measures have been proposed in order to solve this problem. For example, DE 10 2007 032 498 A1 proposes to support a work spindle in a spindle housing on a ring that is springably suspended, as well as axially displaceable and tiltable, relative to the spindle housing. This makes it possible to prevent the bearing of the work spindle from being overloaded in the axial and in the radial direction. However, spring elements can only develop a damping characteristic within a certain deflection range. This means that a severe collision can still occur when the work spindle is inadvertently displaced over greater distances.
A metrological approach is disclosed in DE the 103 51 347 A1. In this case, the tool clamping device of a work spindle is equipped with a pressure or force sensor that is primarily intended to measure the stress on the work spindle occurring during normal operation. An auxiliary function of such a sensor also makes it possible to detect an overload due to a collision with a work piece. In this case, the spindle motion can be stopped and inspection or repair measures can be initiated. However, it cannot be ensured that damages are reliably prevented due to the unavoidable latency in detecting the collision.
In order to prevent collisions of the type in question in the first place, it has also been proposed to carry out a computer-assistant comparison of the measured motion sequence of a work spindle with a previously stored model of the processing chamber. According to EP 0 104 409 A2, hazard zones are defined in the processing chamber, wherein the motion of the work spindle is stopped when these hazard zones are reached. However, computer-assisted monitoring devices have the disadvantage of requiring a high technical operating effort. 3D models from the construction need to be loaded into the machine control. Every motion of the machine simultaneously needs to be compared with this model. It is therefore only sensible to utilize such systems in more complex processing centers.
Another approach to preventing collisions is the optical monitoring of the processing chamber, e.g., as proposed in JP 2006-102923 A. In this case, the processing chamber is monitored with cameras, and the contours of objects present in the processing chamber are determined from the recorded images with pattern recognition algorithms. These contours are then compared with the intended motion path of the work spindle or a tool clamped therein in order to predict and prevent a possible collision by stopping the spindle motion in a timely fashion. Optical systems in combination with image analysis methods likewise require a high computational effort for carrying out the monitoring with real-time capability. Systems of this type quickly reach the limits of electronic processing capabilities during faster displacements. In addition, the image analysis is impaired by lubricant splashing and chips flying around the processing chamber.